Thermal processing systems, such as laser and plasma arc systems, are widely used in the cutting, welding, heat treating, and processing of metallic materials. A laser-based apparatus generally includes a nozzle through which a gas stream and laser beam pass to interact with a workpiece. Both the beam and the gas stream exit the nozzle through an orifice and impinge on a target area of the workpiece. The laser beam heats the workpiece. The resulting heating of the workpiece, combined with any chemical reaction between the gas and workpiece material, serves to heat, liquefy and/or vaporize a selected area of workpiece, depending on the focal point and energy level of the beam. This action allows the operator to cut or otherwise modify the workpiece.
In general, a thermal processing system can include a laser-based or plasma-based torch, an associated power supply, a gas console, a positioning apparatus, a cutting table, a torch height control, and an associated computerized numeric controller (CNC).
In operation, a user places a workpiece on the cutting table and mounts the torch on the positioning apparatus, which provides relative motion between the tip of the torch and the workpiece to direct the laser beam or the plasma arc along a processing path. The CNC accurately directs motion of the torch and/or the cutting table to enable the workpiece to be cut to a desired pattern. Position information is returned from the positioning apparatus to the CNC to allow the CNC to operate interactively with the positioning apparatus to obtain an accurate cut path.
A torch height control module sets the height of the torch relative to the workpiece. A lifter, which is controlled by the torch height control module through a motor, moves the torch in a vertical direction relative to the workpiece to maintain a desired processing quality during cutting for a particular application. Typically, capacitive height sensing (CHS) is used to measure the distance between the torch and the workpiece during machining. In a typical implementation of CHS, a single sinusoidal electrical signal is applied to the torch and the capacitive impedance between the torch and the workpiece is measured to derive the distance. CHS models the electrical circuit between the torch and the workpiece as a single capacitor. As a result, CHS does not provide for accurate distance measurements in situations where a plasma or other electrically conductive substance forms or is present between the torch and the workpiece.